1. Field of the Invention
The present invention relates to an optical fiber applicable to a module installed in a part of an optical transmission line or on the optical transmission line in an optical transmission system which carries out WDM communications mainly in a 1.55-xcexcm wavelength band.
2. Related Background Art
WDM (Wavelength Division Multiplexing) communication systems enable large-capacity, high-speed optical communications by transmitting a plurality of signal light components in the 1.55-xcexcm wavelength band (1.53 xcexcm to 1.57 xcexcm). Since optical transmission systems carrying out such WDM communications preferably have a low dispersion in the 1.55-xcexcm wavelength band so as to be able to transmit signal light in a wide wavelength band, a dispersion-shifted optical fiber whose zero-dispersion wavelength is shifted to the 1.55-xcexcm wavelength band (DSF: Dispersion Shifted Fiber) has been utilized in their optical transmission lines.
If the dispersion in the 1.55-xcexcm wavelength band is substantially zero, however, then four-wave mixing, which is a kind of nonlinear optical phenomena, may occur, whereby the signal light at the time of reception is likely to deteriorate (see, for example, H. Taga, et al., OFC""98, PD13) Therefore, a dispersion-shifted optical fiber whose zero-dispersion wavelength is further shifted to the longer wavelength side so that the dispersion at a wavelength of 1.55 xcexcm is set to about xe2x88x922 ps/nm/km (no zero-dispersion wavelength exists in the signal wavelength band) (NZ-DSF: Non-zero Dispersion Shifted Fiber) has conventionally been employed in optical transmission lines, so as to suppress the four-wave mixing. Since the above-mentioned NZ-DSF has a negative dispersion in the 1.55-xcexcm wavelength band, there are cases where a dispersion-compensating optical fiber having a positive dispersion in the 1.55-xcexcm wavelength band is employed in an optical transmission line together with the NZ-DSF (see, for example, M. Suzuki, et al., OFC""98, PD17).
As the above-mentioned dispersion-compensating optical fiber, optical fibers defined by G652 and G654 standards of ITU-T, for example, have been known. The optical fiber of G652 standard is a regular optical fiber constituted by a core region made of Ge-doped silica and a cladding region made of pure silica. This optical fiber of G652 standard has a zero-dispersion wavelength in a 1.3-xcexcm wavelength band and a dispersion of about 17 ps/nm/km in the 1.55-xcexcm wavelength band. On the other hand, the optical fiber of G654 standard has a dispersion of 20 ps/nm/km or less in the 1.55-xcexcm wavelength band. Further, an optical fiber, constituted by a core region made of pure silica and a cladding region made of F-doped silica, having a dispersion of about 18 ps/nm/km in the 1.55-xcexcm wavelength band is also used as a dispersion-compensating optical fiber.
Since a conventional optical transmission line thus constituted by the NZ-DSF and the dispersion-compensating optical fiber has a positive dispersion slope as a whole, though the dispersion becomes zero in one wavelength in the 1.55-xcexcm wavelength band, it does not become zero in the other wavelength regions. Therefore, in order to compensate for the residual dispersion in the other wavelength regions, the signal light in the other wavelength regions is demultiplexed in a base station or the like, so that the dispersion of each signal light component is compensated for by use of a dispersion-compensating optical fiber of G652 or G654 standard. Here, the dispersion slope is given by the gradient of the curve indicating the dependence of the dispersion upon wavelength.
As a result of studies concerning the above-mentioned prior art, the inventors have found the following problems. Namely, since the upper limit of dispersion in the 1.55-xcexcm wavelength band exceeds 20 ps/nm/km in the above-mentioned dispersion-compensating optical fiber of G654 standard, it is needed to be elongated so as to compensate for the negative dispersion inherent in the NZ-DSF in the 1.55-xcexcm wavelength band. Also, in optical fibers having a simple step-like refractive index profile composed of a core region and a cladding region, the upper limit of dispersion is determined according to the upper limit of cutoff wavelength, whereby it is difficult to enhance the dispersion in the 1.55-xcexcm wavelength band.
In order to overcome the problems such as those mentioned above, it is an object of the present invention to provide an optical fiber which has a large positive dispersion in the 1.55-xcexcm wavelength band, and compensates for the negative distribution inherent in the NZ-DSF in the 1.55-xcexcm wavelength band.
The optical fiber according to the present invention comprises a core region extending along a predetermined axis, and a cladding region disposed at the outer periphery of the core region. The cladding region has a depressed cladding structure comprising an inner cladding which is a region disposed at the outer periphery of the core region, and an outer cladding which is a region disposed at the outer periphery of the inner cladding and has a refractive index lower than that of the core region but higher than that of the inner cladding. Also, in this optical fiber, the relative refractive index difference of the core region with respect to the outer cladding is at least 0.30% but not greater than 0.50%, and the relative refractive index difference of the inner cladding with respect to the outer cladding is at least xe2x88x920.50% but not greater than xe2x88x920.02%. At a wavelength of 1.55 xcexcm, the optical fiber has a dispersion greater than 18 ps/nm/km and an effective cross-sectional area Aeff of at least 70 xcexcm2.
As indicated in Japanese Patent Application Laid-Open No. 8-248251 (EP 0724171 A2), the effective cross-sectional area Aeff is given by the following expression (1):                               A          eff                =                  2          ⁢                                                    π                ⁡                                  (                                                            ∫                      0                      ∞                                        ⁢                                                                  E                        2                                            ⁢                      r                      ⁢                                              ⅆ                        r                                                                              )                                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      ⅆ                    r                                                              )                                                          (        1        )            
where E is the electric field accompanying the propagated light, and r is the radial distance from the core center.
Since this optical fiber has a large dispersion in the 1.55-xcexcm wavelength band as such, a short length is sufficient when compensating for the negative dispersion inherent in the NZ-DSF in the 1.55-xcexcm wavelength band. As a consequence, it is favorable in that, when the optical fiber is wound at a predetermined diameter so as to form a module, the resulting module can be made smaller. Also, since the effective cross-sectional area at the wavelength of 1.55 xcexcm is large, nonlinear optical phenomena can effectively be restrained from occurring. In addition to the characteristics mentioned above, the optical fiber according to the present invention preferably has a dispersion of 20 ps/nm/km or greater at the wavelength of 1.55 xcexcm. Since this optical fiber has a greater dispersion in the 1.55-xcexcm wavelength band, it can be made shorter when compensating for the negative dispersion inherent in the NZ-DSF in the 1.55-xcexcm wavelength band, whereby it becomes easier to reduce the dimensions of a dispersion-compensating module to which the optical fiber is applied. In particular, for realizing various characteristics at the wavelength of 1.55 xcexcm, each of the optical fibers having the configurations mentioned above preferably satisfies the relationships of:
2.0xe2x89xa62b/2axe2x89xa66.0 
8.3xe2x89xa62axe2x89xa613.0 
where 2a (unit: xcexcm) is the outside diameter of the core region, and 2b (unit: xcexcm) is the outside diameter of the inner cladding.
The optical fiber according to the present invention may have a configuration comprising a core region which extends along a predetermined axis and has an outside diameter of at least 9.5 xcexcm but not greater than 13.0 xcexcm, and a cladding region having a refractive index lower than that of the core region. In such a configuration, the relative refractive index difference of the core region with respect to the cladding region is at least 0.3% but not greater than 0.5%. Also, the dispersion at the wavelength of 1.55 xcexcm is at least 20 ps/nm/km, and the effective cross-sectional area Aeff at the wavelength of 1.55 xcexcm is at least 70 xcexcm 2. Since this optical fiber also has a large dispersion in the 1.55-xcexcm wavelength band, a short length is sufficient when compensating for the negative dispersion inherent in the NZ-DSF in the 1.55-xcexcm wavelength band. Also, since the effective cross-sectional area at the wavelength of 1.55 xcexcm is large, nonlinear optical phenomena are effectively restrained from occurring.
Preferably, each of the optical fibers having various configurations mentioned above has a transmission loss of 0.215 dB/km or less at the wavelength of 1.55 xcexcm when wound like a coil at a diameter of 60 mm, and a polarization mode dispersion of 0.25 psxc2x7kmxe2x88x92xc2xd or less at the wavelength of 1.55 xcexcm. In this case, sufficient characteristics can be obtained in the optical fiber according to the present invention even in a configuration in which it is wound like a coil so as to form a module.
As a further preferred optical characteristic, the optical fiber according to the present invention has an effective cross-sectional area Aeff of 90 xcexcm2 or greater. Also, this optical fiber has a cutoff wavelength of 1.4 xcexcm or greater at a fiber length of 2 m. Further, this optical fiber has a transmission loss of 0.180 dB/km or less at the wavelength of 1.55 xcexcm.
The inventors have experimentally confirmed that providing a carbon coating on the surface of the optical fiber according to the present invention is effective in preventing the optical fiber from breaking.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.